1. Field of the Invention
The present invention relates to a plan position indicator (PPI) display apparatus for displaying information of a radar, sonar, etc. on a circular screen of a cathode-ray tube (CRT) through electron beam sweep based on a polar coordinate deflection system (a PPI system).
2. Description of the Prior Art
A conventional PPI display apparatus which can display information of a radar, sonar, etc. is shown in FIG. 1, for example.
Referring to FIG. 1, an antenna angle generator 10 ordinarily uses a resolver or the like to generate a sine signal (sin.theta..sub.1) and a cosine signal (cos.theta..sub.1) proportional to an antenna angle .theta..sub.1. An electronic bearing marker angle generator 12 generates a bearing marker angle signal representing the angle of a bearing marker which is displayed on the CRT screen. It ordinarily uses a resolver or sine/cosine function generating potentiometer to generate signals sin.theta..sub.2 and cos.theta..sub.2 proportional to bearing angle .theta..sub.2 which is set by a manual operation signal.
The outputs from the antenna angle generator 10 and the electronic bearing marker angle generator 12 are inputted to signal selection switches 14 and 16. The signal selection switches 14 and 16 are switched under the control of an externally supplied switching control signal to selectively supply the sine and cosine signals from either the antenna angle generator 10 or the electronic bearing generator 12 to sweep signal generators 18 and 20. The sweep signal generators 18 and 20 can integrate the sine and cosine signals supplied as DC voltage signals from the signal selection switches 14 and 16 to produce sweep signals in the form of saw-toothed signals. The sweep signals are inputted to sweep current amplifiers 22 and 24. The sweep current amplifiers 22 and 24 supply amplified deflection currents to perpendicular deflection coils 26 and 28 of a CRT, whereby the image of a radar, sonar, etc. or bearing marker is displayed on the PPI display through polar coordinate deflection of an electron beam.
The sweep signal generators 18 and 20 receive externally supplied X- and Y-axis (i.e. horizontal and vertical) off-center control signals, and has an off-center function capable of shifting the sweep start point of an antenna angle sweep signal or that of a bearing marker angle sweep signal independently to a given point on the CRT screen. More specifically, off-center control signals for the antenna angle deflection signal and bearing marker angle deflection signal are supplied on a time sharing basis in a synchronous relation to a switching control signal externally supplied to the signal selection switches 14 and 16. Thus, the sweep start points for the antenna information and the bearing marker angle information can be set independently.
In the prior art PPI display system as described above, however, the sine and cosine signals representing the antenna angle .theta..sub.1 and the sine and cosine signals representing the bearing marker angle .theta..sub.2 are generated as analog signals from a resolver or the like. In this case, the precision of the sine and cosine signals depends on the mechanical and electrical precisions of the transmitter such as a resolver. These precisions can only be increased with an increase in cost. In addition, limitations are imposed on the precisions, so that it is difficult to increase the resolution of a target detected by the radar, sonar, etc. beyond a certain limit.
Meantime, there are various kinds of display systems in the PPI display of a radar. In one system, for instance, a head of ship called "HEAD UP" is set at the top of PPI screen for displaying information. In another system, a north bearing in a map or chart called "NORTH UP" is set at the top of PPI screen. In still another system, an intended course of ship called "COURSE UP" is set at the top of PPI screen.
In case of "NORTH UP", it is necessary to detect a deflection angle between the bearing of the ship head and the north bearing and continously correct the antenna angle of radar varing from hour to hour with the deflection angle. In case of "COURSE UP", similarly, it is necessary to detect a deflection angle between the bearing of the ship head and the preset course and correct continuously the antenna angle of radar with the deflection angle.
However, there have been problems in the aspects of precision and cost to add continuously such corrections with the deflection angle to the antenna angle which is a conventional analog signal and further to obtain the sine and cosine signals corresponding to the corrected angle.
In addition, all the signals involved are processed as analog signals, and the processing is susceptible to the influence of temperature. Electric temperature compensation is of course provided. However, since the system is used under comparatively severe conditions such as in ships, the influence of temperature cannot be perfectly compensated for, and the stability of the system is inferior.